(1) Field of the Invention
The present invention relates to a method and to a device making it possible to perform a check of the state of health of a turbine engine arranged on an aircraft, said aircraft being provided with at least one turbine engine.
(2) Background Art
Such an aircraft may be a rotary-wing aircraft, also known as a rotorcraft. However, the aircraft may also be an aircraft not provided with such a rotary wing.
A rotorcraft is piloted while monitoring many instruments on the instrument panel. Most of the instruments represent operation of the power plant of the rotorcraft.
For physical reasons, there are numerous limits that the pilot needs to take into account at each instant while flying. These various limits generally depend on the stage of flight and on outside conditions.
Most presently-constructed rotorcraft are fitted with at least one turboshaft engine having a free turbine for driving a rotary wing in rotation. Drive power is then extracted (taken off) from a low pressure stage of each free turbine, which stage is mechanically independent of the assembly comprising the compressor and the high pressure stage of the engine. Each free turbine of the engines operates at a speed of rotation lying in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a speed-reduction gearbox is needed in the connection with the main rotor of the rotorcraft since its speed of rotation lies substantially in the range 200 rpm to 400 rpm: this is referred to as the “main power transmission gearbox” or “MGB”.
Thermal limits on the engine and torque limits on the MGB serve to define, for example, three normal utilization ratings for the engine:
Among known ratings, mention may be made of the following:                the takeoff rating which associates a maximum takeoff power PMD with a duration of utilization of 5 minutes;        the maximum continuous rating associating a maximum continuous power PMC with an unlimited utilization duration; and        the transient rating associating a maximum transient power PMT with a limited utilization duration of about 20 seconds.        
There also exist super-contingency ratings for aircraft having at least two engines, these ratings being for use when one of the engines fails:                a first contingency rating associating a super-contingency power with a duration of about thirty consecutive seconds known as 30 sec OEI (for one engine inoperative), this first contingency rating being usable on about three occasions during a flight;        a second contingency rating associating a maximum contingency power with a utilization duration of about two minutes, known as 2 min OEI; and        a third contingency rating associating an intermediate contingency power OEIcont with a utilization duration extending to the end of a flight after the engine has failed, for example.        
By calculation or by testing, the engine manufacturer draws up available power curves for an engine as a function of altitude and of temperature, with this being done for each of the above-defined ratings. Similarly, the engine manufacturer determines the life of the engine and the guaranteed minimum power for each rating; this guaranteed minimum power corresponding to the power that the engine will deliver when it has reached the end of its life, such an engine being referred to by convenience as an “aging engine” in the remainder of the text below.
In order to verify that the engine is operating properly, it is thus advisable to perform a health check so as to make sure that the engine has performance greater than or equal to the performance of an aging engine. In addition, the health check may serve to guarantee contingency power in the event of failure of an engine.
Such a health check consists in determining the power margin of an engine relative to a minimum power as measured, for example, on a test bench. If the power margin is positive, the engine remains capable of delivering the required power. Otherwise, maintenance action should be undertaken to re-establish the performance of the engine.
The health check may be performed by determining a power margin as such, or by determining a margin of a monitoring parameter of the engine relative to a measurement taken on a test bench.
In general, an operating margin is determined that can be a power margin or a monitoring parameter margin.
In particular, two monitoring parameters are used to check the performance of an engine.
Since the engine is provided with a turbine assembly, one monitoring parameter may be the temperature of the gas flowing through said assembly.
In particular, since a high-pressure turbine is disposed upstream from a free turbine, a first monitoring parameter may be the temperature of the gas at the inlet to the high-pressure turbine, known as the Turbine Entry Temperature or “TET” by the person skilled in the art.
The blades of the high pressure turbine of the engine are subjected to centrifugal force and to the temperature TET. Beyond a certain level, the component material of the turbine blades is subjected to creep, resulting in expansion that lengthens the turbine blades. Thus, the turbine blades might touch the casing of the high-pressure turbine and thus be degraded. The temperature TET is thus associated directly with degradation of the engine.
Nevertheless, since the temperature TET is very difficult to measure because of its relatively non-uniform nature; the first monitoring parameter may be the temperature of the gas at the entry to the free turbine, known to the person skilled in the art as “T45”. Since this temperature T45 is a good indicator of the temperature TET, it is representative of the degradation of the engine.
A first monitoring parameter is thus the temperature of an assembly having at least one turbine, this temperature possibly being the temperature TET of the gas at the inlet to the high-pressure turbine or the temperature T45 of the gas at the inlet to the free turbine.
A health check may consist in determining a temperature margin relative to a minimum reference temperature.
Furthermore, a second monitoring parameter relates to the power delivered by the engine or to the torque from its shaft, where power and shaft torque are mutually dependent. Nevertheless, the speed of rotation of the gas generator of the engine, known as “Ng” by the person skilled in the art, is also linked with the power delivered by the engine, so the second monitoring parameter that can be used is this speed of rotation of the gas generator.
Consequently, checking the state of health of the engine consists, for example, in:                measuring the first monitoring parameter and then verifying that the current power value is greater than or equal to the power value that an aging engine would deliver under the same conditions; or        measuring the second monitoring parameter and then verifying that the current power value is greater than or equal to the power value that would be delivered by an aging engine under the same conditions.        
It is also possible to measure a drive torque and a speed of rotation so as to deduce therefrom the power developed by an engine. For example, the torque exerted on an outlet shaft driven by the free turbine and the speed of rotation of said outlet shaft are measured. Alternatively, it is possible to measure the speed of rotation of the main rotor, and to multiply that speed of rotation by a coefficient corresponding to the speed reduction ratio existing between the speed of rotation of the outlet shaft and the speed of rotation of the main rotor.
By way of an alternative, the flight is flown with a given speed of rotation Nr of the main rotor, and said torque is measured. By means of charts, a torque margin is deduced.
The health check should be performed rigorously because if it is negative, i.e. if the above-mentioned verifications do not give satisfactory results, it has a non-negligible impact on any downtime of the aircraft and on the costs of maintaining said aircraft.
In order to compare the results of measurements taken in flight with measurements taken on a test bench, it is preferable for the in-flight measurement conditions and for the test-bench measurement conditions to be as close as possible.
The measurements taken on a test bench are taken under thermally stable conditions.
In order to perform a health check in flight, the pilot places the aircraft in a particular stage of flight such as level flight at altitude and speed that are stabilized for several minutes. The pilot can then launch a manual action requesting collection of the monitoring parameters necessary for the health check, and then calculation of at least one operating margin.
That method then includes: a step of stabilizing the aircraft, a step of acquiring at least one value for a monitoring parameter, and a step of evaluating an operating margin. A step of maintaining the engine may then be undertaken depending on the results of the evaluation step.
Document FR 2 899 640 describes a method of performing a health check on at least a first turbine engine of a rotorcraft, that rotorcraft being provided with first and second engines.
Document U.S. Pat. No. 7,487,029 proposes a method of monitoring the performance of a turbine engine for maintenance planning purposes.
Document U.S. Pat. No. 8,068,997 proposes a method of analyzing performance of a turbine engine in real time by using a transfer function and statistical tools.
Document EP 1 970 786 proposes a method of analyzing the operational data of an engine and potential faults reflected in such data.
Document EP 2 202 500 discloses a system for assisting maintenance and operation of a gas turbine.
Document FR 2 902 408 is also known.